Method For Sequestering Carbon Dioxide In Geological Formations

ABSTRACT

A method for enhancing the storage of carbon dioxide underground. The method comprises locating a geological formation comprising a first storage reservoir having a first surface and a second surface, a first seal layer adjacent the first surface of the first storage reservoir, a second seal layer adjacent the second surface of the first storage reservoir, and a second storage reservoir adjacent the first seal layer opposite the first storage reservoir; directing carbon dioxide into the first storage reservoir, the carbon dioxide being at a first pressure within the first storage reservoir; directing a fluid into the second storage reservoir, the fluid being at a second pressure within the second storage reservoir; and maintaining the second pressure at generally the same or larger pressure than the first pressure of the carbon dioxide to reduce leakage of CO 2 .

PRIORITY

The present patent application claims priority from U.S. Ser. No.61/226,914 filed on Jul. 20, 2009, the entire contents of which areincorporated herein by reference.

FIELD

The present patent application relates to sub-surface sequestration and,more particularly, to sequestration of carbon dioxide in geologicalformations.

BACKGROUND

Carbon dioxide can be contained in subsurface geological units(formations) by a process known as CO₂ sequestration or enhancedhydrocarbon recovery. The CO₂ stored underground may originate fromcarbon emissions or from other naturally occurring sources of CO₂. TheCO₂ is delivered to the storage site via normal transport from pipelinesin a liquefied state or can be liquefied on site and is then pumpedunderground into a geological layer that has adequate porosity andpermeability for storing CO₂, referred to as a storage reservoir. Thestorage reservoir acts as a container for storing the CO₂ whengeological seal layers, e.g., overlying seal, underlying seal, adjacentseal or fault seal, are positioned above and below and adjacent thestorage reservoir. The storage reservoir is typically composed of porousrocks with higher porosity and permeability, for example sandstones,carbonates and chalks. The geological seal layer is typically composedof rocks that are less permeable, for example shales, salts ofanhydrites, and low porosity limestones of sandstones.

The seal or cap rock will have a certain capacity to hold the CO₂ withinthe storage reservoir without leaking. These properties for containingCO₂ largely relate to the seal capacity which is related to a number ofvariables including but not limited to porosity, permeability, andinterfacial tension of the different molecules of various fluids oil,gas CO₂ and water in combination, pressure, temperature andgeomechanical properties of rock strength. Accordingly, the naturalcharacteristics of the cap rock formation are relied on for sealing andcontaining the CO₂. When the CO₂ is injected and stored in the storagereservoir it increases the pressure on geological seal layers, and ifthe pressure is increased too much it will breach the cap rock bydilation or leakage of mechanically fracturing the rock. A breach in thecap rock will release the CO₂ from containment and contaminate othergeological zones, which may include the atmosphere, and lead to seriousenvironmental consequences. Accordingly, underground sequestration ofCO₂, while effective for storing CO₂, faces the problem of cap rockfractures or leakage that can allow CO₂ to leak from the storagereservoir.

Herein, a method is disclosed that enhances the storage of carbondioxide underground by balancing the pressure the CO₂ applies to theseal rock by pressurizing at least one additional storage reservoiradjacent to the seal rock with the pressure of a fluid directed intothat additional storage reservoir. The pressure of the fluid ismaintained at generally the same pressure of the CO₂ within its storagereservoir or at a pressure larger than the pressure of the CO₂ withinits storage reservoir. This balance of pressure exerted on the seallayer reduces the occurrence of a breach in the seal layer for longer,safer CO₂ storage.

SUMMARY

In one aspect, a method for enhancing the storage of carbon dioxideunderground is disclosed. The method comprises locating a geologicalformation that includes, but is not limited to, a first storagereservoir having a first surface and a second surface, a first seallayer adjacent the first surface of the first storage reservoir, asecond seal layer adjacent the second surface of the first storagereservoir, and a second storage reservoir adjacent the first or thesecond seal layers opposite the first storage reservoir; directingcarbon dioxide into the first storage reservoir, the carbon dioxidebeing at a first pressure within the first storage reservoir; directinga fluid into the second storage reservoir, the fluid being at a secondpressure within the second storage reservoir; and maintaining the secondpressure at generally the same or larger pressure than the firstpressure of the carbon dioxide to reduce the occurrence of a breach inthe first seal layer and/or the second seal layer.

In another aspect, the geological formation includes the second storagereservoir adjacent the first seal layer, and a third storage reservoiradjacent the second seal layer. The method also includes directing afluid into the third storage reservoir, with the fluid in the thirdstorage reservoir at a third pressure, and maintaining the thirdpressure at generally the same or larger pressure than the firstpressure of the carbon dioxide.

In yet another aspect, the method includes monitoring the pressure inthe first storage reservoir and the second storage reservoir, and thethird storage reservoir when present.

In yet another aspect, the fluid in the second storage reservoir and thethird storage reservoir may be the same or different. The fluid may bewater or a composition comprising water from an above ground or anunderground source.

Other aspects of the disclosed method for sequestering carbon dioxide ingeological formations will become apparent from the followingdescription, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a geological formationincluding a bore placing a carbon dioxide source in fluid communicationwith a first storage reservoir and a second bore placing a fluid sourcein fluid communication with both a second storage reservoir and a thirdstorage reservoir.

FIGS. 2A and 2B are flow charts of methods for enhancing the undergroundstorage of carbon dioxide.

FIG. 3 is a side cross-sectional view of a geological formationincluding a fourth storage reservoir.

FIG. 4 is a side cross-sectional view of a geological formationincluding an extraction well for accessing an underground fluid sourcefound in a fourth storage reservoir, the extraction well being in fluidcommunication with the second bore of FIG. 3.

FIG. 5 is a side cross-sectional view of a geological formationincluding an extraction well for accessing a fluid free of CO₂ presentin the first storage reservoir, the extraction well being in fluidcommunication with the second bore of FIG. 3 for distribution to thesecond and/or third storage reservoirs.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention may have been simplified to illustrate elements that arerelevant for a clear understanding of the present invention. Those ofordinary skill in the art will recognize that other elements may bedesirable and/or required in order to implement the underground storageof carbon dioxide. However, because such elements are well known in theart, and because they do not facilitate a better understanding of thepresent invention, a discussion of such elements is not provided herein.It is also to be understood that the drawings included herewith onlyprovide diagrammatic representations of the presently preferredstructures of the present invention and that structures falling withinthe scope of the present invention may include structures different thanthose shown in the drawings. Reference will now be made to the drawingswherein like structures are provided with like reference designations.

Referring to FIGS. 1 and 2A-2B, a method 200 is disclosed for enhancingthe storage of carbon dioxide underground in a suitable geologicalformation by increasing the sealing capacity of seal layers (e.g., caprock) by controlling the pressures applied to the seal layers. Themethod 200 may include locating 202 a geological formation 100 thatincludes, but is not limited to, a first storage reservoir 102 having afirst surface 104 and a second surface 106, a first seal layer 108adjacent the first surface 104 of the first storage reservoir 102, asecond seal layer 110 adjacent the second surface 106 of the firststorage reservoir 102, and a second storage reservoir 112 adjacent thefirst seal layer 108 opposite the first storage reservoir. In analternate embodiment, the second storage reservoir may be adjacent thesecond seal layer 110 opposite the first storage reservoir. The methodincludes directing 204 carbon dioxide into the first storage reservoir102, the carbon dioxide being at a first pressure within the firststorage reservoir and directing 206 a fluid, which may be a liquid or agas, into the second storage reservoir 112, the fluid being at a secondpressure within the second storage reservoir. The method also includesmaintaining 208 the second pressure at generally the same or a largerpressure than the first pressure of the carbon dioxide to reduce theoccurrence of a breach in the first and/or the second seal layers.

As shown in FIG. 1, the geological formation 100 includes a thirdstorage reservoir 114 adjacent the second seal layer 110, a third seallayer 116 adjacent the second storage reservoir 112 opposite the firstseal layer 118 and a fourth seal layer 118 adjacent the third storagereservoir 114 opposite the second seal layer 110.

A CO₂ source 120 may be connected to a bore 122 placing the CO₂ in fluidcommunication with the first storage reservoir 102. The bore 122 mayinclude a port 124 through which the CO₂ enters the first storagereservoir. A fluid source 130 may be connected to a bore 132 placing thefluid in fluid communication with the second storage reservoir 112and/or the third storage reservoir 114. The bore 132 may include a port134 through which the fluid enters the second storage reservoir 112 anda second port 136 through which the fluid enters the third storagereservoir 114. The CO₂ and the fluid may be pumped or injected intotheir respective storage reservoirs under pressure.

Referring to FIG. 2B, an alternate method may include locating 202′ ageological formation that includes the third storage reservoir 114 asdescribed for FIG. 1 in addition to the first and second storagereservoir. This method may include the following additional steps:directing 210 a fluid into the third storage reservoir 114, with thefluid in the third storage reservoir at a third pressure, andmaintaining 208′ the second pressure and third pressure at generally thesame or larger pressure than the first pressure of the carbon dioxide.The method may also include monitoring 212 the pressures in the first,the second, and the third storage reservoirs so that their respectivepressures can be adjusted or maintained as needed to keep an overallbalance of pressure within the geological formation, in particular thepressure acting on each seal layer.

The term “fluid,” as used herein, includes any material that is capableof flowing into the porous, highly permeable rocks in a storage layer,especially gases, liquids, and solutions, suspensions, or dispersions ofmaterials in gases or liquids. The fluid may be water or a compositionincluding water and some chemicals (or a combination of fluids), inparticular chemicals or other fluids that can further enhance the seal.The fluid may also be a gas or an oil brine from oil and gas fieldproduction.

The fluid can be modified to change its temperature or pH to minimize orcontrol rates of acidification or other chemical reactions, byintroducing seal-enhancing agents, by manipulating the density of thefluid to enhance the seal of the seal layers. The fluid density can bedirectly manipulated by addition of solids, thereby creating solutiondensities to increase pressure and addition of gas constituents tofurther create a series of various density materials, which both can beused to control the reservoir and seal system. In one embodiment,lost-circulation material may be added to the fluid to enhance the seal.Lost-circulation material refers to substances added to drilling fluidswhen drilling fluids are being lost to the formations downhole forexample fibrous substances such as cedar bark, shredded cane stalks,mineral fiber and hair, flaky substances such as mica flakes and piecesof plastic or cellophane sheeting, or granular substances such as groundand sized limestone or marble, wood, nut hulls, Formica, corncobs,cotton hulls and swelling clays.

The fluid and the CO₂ may be injected into their respective storagereservoirs by drilling wells and perforating the adjacent reservoir andthen pumping the fluid or CO₂ into the reservoir to increase thepressure. The fluid and the CO₂ may be injected sequentially orsimultaneously. The fluid is extracted from other adjacent reservoirsnot in contact with the CO₂ storage reservoir. Alternately, the fluid isobtained from an above ground source. The injection rate of the fluidmay be controlled by specific placement of well injectors, which injectfluids to increase pressure in the fluid's storage reservoir. Thisincrease in pressure outside of the CO₂ storage reservoir, i.e., in thefluid's storage reservoir, results in less differential in pressureacross the cap rock or seal layer between the CO₂ storage reservoir andthe fluid's storage reservoir, which provides the advantage of enhancedsealing capacity within the geological formation, and hence less risk ofCO₂ leaks.

Referring now to FIG. 3, there are four reservoir zones, A-D, separatedby seals with 4 seal layers shown as Seal 1, Seal 2, Seal 3, and Seal 4.Increasing the pressure in Reservoir A and C by injecting a fluid into Aand C through perforations in well bores increases the pressure in A andC and provides a decreased differential in pressure across Seal 2 andSeal 3. Reservoir

B is the reservoir into which the CO₂ is injected and stored. Seal 2 andSeal 3 define the capacity to store the CO₂ in Reservoir B. The CO₂ isbeing injected by well bores perforated into Reservoir B from a sourceof CO₂ at the surface. As the pressure is increased in Reservoir B withCO₂, this method increases proportionally the pressure in Reservoir Aand C such that the differential pressure across the Seal 2 and Seal 3stays at a minimum to maximize the sealing capacity of Seals 2 and 3. Bymaintaining pressure in balance it is possible to more safely store theCO₂ for a longer period of time. The pressure in each reservoir is notedon the diagram by P1, P2, P3, P4. Here, P1, P2, and P3 are kept as closeas possible to the same pressure value to provide enhanced seal capacityto keep the CO₂ contained. Without the increased pressure in ReservoirsA and C the pressure in Reservoir B would increase until at some pointthe seal will fail, much sooner than by maintaining the pressure balanceoutlined by this process. In an alternate embodiment, P1 and P3 may begreater than P2 to further strengthen the seal.

In the embodiment of FIG. 4, the same geological formation of FIG. 3 isshown, except that the fluid for injection into Reservoirs A and C isfrom a subsurface reservoir deemed appropriate and safe, such asReservoir D. The fluid is extracted from Reservoir D by an extractionwell that pumps the fluid from the subsurface reservoir to the injectorwell connected to Reservoirs A and C.

Alternately, as shown in FIG. 5 the fluid for injection into ReservoirsA and C can also come from Reservoir B itself. Here again the samegeneral geological formation of FIG. 3 is shown, except that anextraction well connected to Reservoir B is shown. The extraction wellshould be connected to Reservoir B at a “down dip” or a lower structuralpoint in Reservoir B where CO₂ is not present, so that the fluid takenfrom Reservoir B does not contain CO₂. This removal of fluid fromReservoir B has an added benefit in that it can lower the pressure inReservoir B thereby further helping to maintain the pressure balanceacross the entire system.

While FIGS. 1-5 refer to an example geological formation with three tofour reservoirs and four seal layers, one skilled in the art willappreciate that geological formation may have any combination ofreservoirs, seal layers, and other geological layers. The geologicalformation can be selected for storage of the CO₂ below or above existingoil or gas fields.

The method disclosed herein also minimizes the pressure differentialacross the geological formation storing the CO₂ to maintain theintegrity and maximize the effectiveness of the seal layers. Wheninjecting fluids into their respective reservoirs the pressure isincreased thereby affecting the pore pressure in the geologicalformation, it is this pore pressure that is controlled or maintained bythe balance of the pressure in the storage reservoirs. The overburdenpressure minus the pore pressure is the effective stress, which accountsfor physical behavior of the rock formations. If pore pressure increasesto close to the overburden pressure which is the pressure caused by theoverlying rock column and fluids, it can cause the rock to fail as itreaches the fracture pressure. This method controls the pore pressure inthe CO₂ storage reservoir rocks and the adjacent formations such thatthe pore pressure in the system does not approach the formation fracturepressure.

This same systematic control of pressure can be used to controlpressures across fault zones thereby also enhancing the sealing andretention capability of a fault. The method is to then balance thepressure on either side of the fault by the same injection of fluids.

In yet another aspect, the method includes monitoring the pressure inthe storage reservoirs. The pressure is monitored in Reservoir A, B, andC by taking periodic pressure measurements with pressure measuringdevices by stopping injection in an injector well and making a pressuremeasurement at specific time intervals or by continuous monitoring. Thepressure in any adjacent used or unused reservoirs can be continuouslymonitored using known techniques to drill wells specifically designed tojust monitor pressure.

Other scientific methods, which support the proper pressure balancedetermination, include calculating the seal capacity of the rockformations by core analysis and/or capillary pressure measurements ofthe subsurface rock samples. The pressure monitoring can also beaccomplished by standard type Bottom Hole Pressure buildups; in this waythere is a monitoring of the entire pressure system over time.Additional monitoring devices such as detection devices for CO₂ in thestorage reservoir and the adjacent reservoirs will also be utilized tomonitor the effectiveness of the process.

The method also includes the cement used around the drilled injectionboreholes, which are used to cement casing in place. The cement used inthis method will contain mineral elements to further enhance the sealingcapacity around wells drilled into the various reservoir and sealsystems. The cement can be modified to use smaller particles and highlypulverized clays, which swell so as to further enhance the sealingcapacity. These cements also take into account the chemical reactions ofclays with CO₂, water, and brines. The method includes using all thesevarious type cements, which are designed to be better sealing cementsfor reservoir containment.

The method also includes using horizontal wells or any subsurface wellfor the injection of CO₂. Using horizontal wells of various lengthsallows for a minimum pressure increase over distance and is better thaninjecting CO₂ from a vertical well. The use of horizontal well injectionfor CO₂ and the seal reservoir balancing injectors is also a part ofthis method. The method of delivery of the injection of fluids utilizescontrolled rates, which are optimal to prevent fracture generation bykeeping pressure around the injection site to less than 90% of thecalculated fracture gradient.

The use of proper pressure and temperature and fluids in the disclosedprocess is unique. The process scheme of using well bores in strategicpositions to provide for injection or removal of fluids in combinationto maintain a balance of pressures in the geological formation andprovide the source for fluids to maintain this balance is unique.

Using the disclosed method, a subsurface storage reservoir formation canbe brought into balance by the proper maintenance of injection ratesand/or removal of the CO₂ and/or the fluid to achieve the desiredbalance of pressure within the CO₂ storage reservoir and the fluidstorage reservoirs to enhance the seal of adjacent cap rock. This methodalso minimizes the damage caused by volume changes in the rockformations that results from pressure changes that change the volume ofthe reservoir rock (e.g., inflate or deflate the reservoir rock) andoverburden the surrounding rock. Accordingly, safer long-termunderground CO₂ storage is provided.

Although various aspects of the disclosed method for sequestering carbondioxide in geological formations have been shown and described,modifications may occur to those skilled in the art upon reading thespecification. The present application includes such modifications andis limited only by the scope of the claims.

1. A method for sequestering a subject fluid comprising: locating ageological formation comprising: a first storage reservoir having afirst surface and a second surface; a first seal layer adjacent thefirst surface of the first storage reservoir; a second seal layeradjacent the second surface of the first storage reservoir; and a secondstorage reservoir adjacent the first or the second seal layers oppositethe first storage reservoir; directing a subject fluid into the firststorage reservoir, the subject fluid being at a first pressure withinthe first storage reservoir; directing a second fluid into the secondstorage reservoir, the second fluid being at a second pressure withinthe second storage reservoir; and maintaining the second pressuregenerally at least at the first pressure.
 2. The method of claim 1,wherein the second storage reservoir is adjacent the first seal layer,the geological formation further comprising a third storage reservoiradjacent the second seal layer, and the method further comprisingdirecting a third fluid into the third storage reservoir, the thirdfluid being at a third pressure within the third storage reservoir; andmaintaining the third pressure generally at least at the first pressure.3. The method of claim 1, wherein the geological formation furthercomprises a third seal layer adjacent the second storage reservoir andopposite the first seal layer, and a fourth seal layer adjacent thethird storage reservoir and opposite the second seal layer.
 4. Themethod of claim 1, further comprising monitoring the pressure in thefirst storage reservoir and the second storage reservoir.
 5. The methodof claim 1, wherein the second fluid comprises water.
 6. The method ofclaim 5, wherein the second fluid includes a seal-enhancing agent. 7.The method of claim 1, wherein at least a portion of the second fluid isfrom the first storage reservoir and is substantially free of CO₂. 8.The method of claim 1, further comprising forming a bore having a portentering the first storage reservoir and a bore having a port enteringthe second storage reservoir.
 9. The method of claim 8, wherein the stepof maintaining the second pressure includes controlling the injectionrates of the second fluid, the subject fluid or a combination thereof.10. The method of claim 1 wherein the subject fluid is carbon dioxide.11. The method of claim 1 wherein the second fluid is a liquid.
 12. Themethod of claim 2 wherein the second fluid and the third fluid are oneand the same.